1. Field of the Invention
This invention relates to a frequency demodulation circuit for demodulating a frequency modulated (FM) signal.
2. Description of the Prior Art
In video tape recorders (VTRs), an FM signal reproduced from a magnetic tape is demodulated by a pulse counting type frequency demodulator which detects zero-crossing points of the FM signal. The demodulated signal obtained by such a frequency demodulator contains the lower sideband components of the FM signal. Among the lower sideband components, components appearing in the base band badly affect the desired signal to cause deterioration of the signal quality. Such undesired components are called "moire" components. As an example, FIG. 13 shows a spectrum diagram of an FM signal obtained by modulating a carrier frequency f.sub.c with a signal frequency f.sub.s and a spectrum diagram of a demodulated signal obtained by demodulating such an FM signal by a double-multiplier pulse counting type demodulator. The lower sideband components J.sub.-1, J.sub.-2, J.sub.-3 . . . , J.sub.-n appearing in the demodulated signal are expressed as: EQU J.sub.-1 =2f.sub.c -f.sub.s, J.sub.-2 =2f.sub.c -2f.sub.s, J.sub.-3 =2f.sub.c -3f.sub.s, . . . , J.sub.-n =2f.sub.c -nf.sub.s
The signal f.sub.s in the base band is obtained by passing the demodulated signal through a low-pass filter (LPF). However, there exist some lower sideband components in the base band as undesired moire components, i.e., the moire components, which degrade the quality of the obtained desired signal. In the case of FIG. 13, the third-order lower sideband component J.sub.-3 becomes the undesired moire component.
To reduce such an undesired moire effect, the carrier frequency is generally selected to be more than 1.5 times the maximum frequency of the modulation signal. The moire effect can be reduced more by using a higher carrier frequency. However, the higher carrier frequency necessarily results in a lower signal to noise ratio (C/N), and the carrier frequency cannot be made high enough to reduce the moire effect adequately.
In home VTRs, the color-under system is used which converts the frequency of the chrominance signal of the video signal into a lower frequency and mixes the low-converted chrominance signal with the frequency-modulated luminance signal for recording.
Supposing that the carrier frequency is designated as f.sub.c and the down-converted chrominance signal frequency as f.sub.L, a cross-modulation component is generated at the frequency of f.sub.c -2f.sub.L during playback due to the frequency characteristics of the electromagnetic conversion system, and a cross-modulation moire component of 2f.sub.L is generated in the reproduced luminance signal after demodulation, causing deterioration of the reproduced picture quality.
Conventionally, the recording current of the chrominance signal is restricted to a low level to reduce the level of the cross modulation moire component below the allowable limit, but this causes a deterioration of the S/N of the chrominance signal.